Whether you are the new guy or girl in school, don't be afraid to be yourself.
Being the new kid sucks. I would know because I was once the new kid. Some people are fortunate that they never have to worry about being the new kid but some people have to deal with it more than they would like to.
It can be scary and uncomfortable being in a new place. Not knowing anyone or where you are going can be the worst feeling. My advice to you is to be as friendly as you can. Some people may not reciprocate the kindness, but at least they can't say that the new kid isn't nice. When you find those people that are nice back to you, try to get to know them. Some people may be putting on an act and don't really care about being your friend, but that's okay. At this point in your life hopefully you are old enough to be able to tell who is going to be the good cookies in the bunch. However, just keep your head held high and remember to make the right choices.
Try to join clubs and sports and get involved. This is a great way for people to meet new friends that have a similar interest as them. It is also a nice way to meet upper or underclassmen. The more people you know in school, the easier it may become for you. Hopefully you can find the right group and can make some worthwhile memories!
Try not to let your academics slip. You may feel stressed and concerned because you are worried and getting used to school and making friends. In the long run your grades are what are going to matter. Ask teachers for help or even go see a guidance counselor if you are having concerns. Remember that all staff members at a school are here to help you.
My last and most important piece of advice is to be yourself. Nobody likes someone that is fake and you don't want to be known as the fake new kid in school. Being yourself will allow you to make good and genuine friends with ease. It will also help you to feel better and more confident that you don't have to put on an act every day when you go to school.
So best of luck to all of the new kids in school. And if you are reading this and you aren't new to a school, be a friend to those that are. That could be you in their situation so think about how you would feel. No one is ever in the wrong for being kind to someone.